Conventionally, a variety of proposals have been made as to a tire improving a cruising performance on an icy or snowy road surface (a snowy road surface or an icy road surface). In order to enhance a cruising performance such as an acceleration performance at the time of startup on an icy or snowy road surface a braking performance at the time of stoppage, it is preferable to increase a component of a side groove along a tread widthwise direction (a so called lag groove) and to increase a percentage of a groove area in a ground surface (a negative rate).
However, if the component of the lag groove is increased too much in order to increase the negative rate, since a stability of block lowers, a driving stability in a dry road surface also lowers. On the other hand, if the content of the lag component is increased without increasing the negative rate, since a groove (a rib groove) component in a circumferential direction along a tire circumferential direction is decreased, a drainage performance also lowers.
In order to ensure compatibility between the cruising performance and the drainage performance in the icy or snowy road surface described above for example, there is proposed a tire in which a lag groove crossing in the tread widthwise direction is formed in a substantially V shape so as to be symmetrical with a tire equator line (refer to Patent Literature 1). According to this tire, the drainage performance is improved by means of a lag groove formed in a substantially V shape so as to be symmetrical with the tire equator line and a main groove that are formed in parallel to each other along the tire circumferential direction at a tread center part including the tire equator line.